1. Field of the Invention
The present invention relates to an apparatus for controlling the power of an AC voltage supplying an electrical consumer by phase control and for reducing harmonics caused by said phase control, particularly within a range of 4 kHz, preferably within a range of the third harmonic, comprising a first switching element connected in series with said consumer and driven by a control device for performing phase control. The invention further relates to a method for reducing harmonics in the range of 4 kHz, preferably in the range of the third harmonic, in a power control performing a phase control.
2. Related Prior Art
An apparatus and a method for power control by means of phase control and phase adjustment, respectively, is for example known from the application DE 198 50 905 A1 of the present applicant. Devices for phase control serve to control the power supplied to an electric consumer by periodically switching the consumer on and off by means of a switching element. Typically, a TRIAC is used as the switching element, which is connected in series with the consumer and which is fired via a control unit.
The control unit comprises a series connection of a resistor and a firing capacitor, the series connection being provided parallel to the TRIAC. The firing signal is tapped off between the resistor and the firing capacitor and is supplied to the control input (gate) of the TRIAC via a series connection of a resistor and a firing element in form of a DIAC. Beside these analogue designed control devices, also integrated circuits are nowadays available which perform this driving function.
The power control via phase control of the AC voltage supplied to the consumer causes undesired harmonics, and there are European standards regulating the level of such harmonics. These standards have to be fulfilled by the power control apparatuses which is the reason why approaches for reducing these harmonics have been proposed in the past.
When reducing the harmonics, particularly the third harmonic plays a special role, therefore, former approaches have concentrated to reduce the third harmonic. An example of an apparatus for reducing the third harmonic is for example disclosed in EP 0 859 452 B1 of the present applicant. In the solution disclosed therein, the phase angle and firing angle, respectively, is changed by the control device around a predetermined value in succeeding full waves. Such an asymmetry in the firing angle results in only slowly increasing even numbered harmonics and highly reduced odd numbered harmonics.
An approach for suppressing noise in a control circuit performing phase control is e.g. disclosed in DE 21 31 750 A. Further circuits performing phase control a disclosed in DE 199 04 615 A1 or DE 43 05 882 A1.
Although the above mentioned approach proved successful in practise and allows to meet the existing standards readily, of course it further exists the request for finding a solution even less expensive. Moreover an apparatus would be desirable, the hum of which is slightly reduced in the range of 25 Hz compared to the apparatus shown in the mentioned document.